1. Field of the Invention
The present invention relates to a combination reflection/transmission-type liquid-crystal display device.
2. Description of the Related Art
Liquid-crystal display devices include a transmission type, in which backlighting is applied to a liquid-crystal cell from the rear and a reflective type, in which external light entering the liquid-crystal display device via the liquid-crystal cell is reflected by the rear surface thereof, the reflected light illuminating the liquid-crystal cell.
In addition, there is a type of liquid-crystal display device which is used as a reflective type display in bright locations and a transmission type of display in dark locations.
Such combination reflective/transmission liquid-crystal display devices are disclosed for example, in the Japanese Unexamined Patent Application publications (JP-A) H10-197865, H10-206844, H10-206846, H10-260402, and H10-260403, in which a translucent sheet is used as a diffuser for backlighting. In the Japanese Unexamined Patent Application publication (JP-A) H10-142601, there is disclosure of the provision of frontlighting at the front surface of the display of a reflective-type liquid-crystal display device, the illumination light provided by this frontlighting or external light being used to illuminate the liquid-crystal cell.
In a liquid-crystal display device, polarized light obtained by passing light through a polarizer is modulated in a liquid-crystal layer. For example, as shown in the typical liquid-crystal display device 1 of FIG. 48, light emitted from a light source 2 strikes a light-absorbing type dichroic linear polarizer 3, the linearly polarized light obtained therefrom then striking a liquid-crystal cell 4.
In the liquid-crystal display device 1 of FIG. 48, polarized light that strikes and passes through the liquid-crystal cell 4 is modulated by application of a voltage to an electrode provided in the liquid-crystal cell 4 so as to change the electric field within the cell, or exits the liquid-crystal cell 4 unmodulated with no electric field, a light-absorbing type dichroic linear polarizer 5 disposed outside the cell passing light of only a particular polarization direction.
Because the light-absorbing type dichroic linear polarizers 3 and 5 pass light polarized in the direction of the transmission axis, and almost completely absorb light polarized in a direction perpendicular to the transmission axis, approximately 50% of the light (nonpolarized light) emitted from the light source 2 is absorbed by the dichroic linear polarizer 3. For this reason, the utilization rate of light in the liquid-crystal display device 1 decreases, making it necessary to have more light from the light source strike the dichroic linear polarizer 3 in order to obtain a sufficient brightness at the surface of the liquid crystal screen.
If the amount of light emitted from the light source 2 is increased, however, there is not only an increase in the electrical power consumed, but also an increase in the heat generated by the light source 2, this leading to the problem of an adverse effect on the liquid crystal in the liquid-crystal cell 4.
Because of this situation, a proposed remedy, as disclosed in a PCT Publication corresponding to JP (Tokuhyo) H4-502524 and in Japanese Unexamined Patent Application publication (JP-A) H6-130424, is a liquid-crystal display device in which a cholesteric liquid crystal layer is used to separate light from a light source by either transmitting or reflecting light of right or left circular polarization, transmitted light of one of the circular polarization directions being caused to strike a liquid-crystal cell, and light of the other of the circular polarization directions being reflected by a reflective sheet as to reverse the direction of rotation thereof, thereby enabling the light to be transmitted through the cholesteric liquid crystal layer, and improving the utilization rate of light.
As disclosed in PCT publication corresponding to JP (Tokuhyo) H9-506985, there is a proposed a liquid-crystal display device in which an extended multilayer film is used to either transmit or reflect nonpolarized light from a light source, thereby dividing the light into two linearly polarized components, the transmitted linearly polarized light being caused to strike a liquid-crystal cell, the reflected linearly polarized light having a polarization direction perpendicular to the transmitted light having its direction of polarization converted by a reflecting sheet, after which it is guided once again so as to strike the extended multilayer film, thereby improving the utilization rate of light.
The liquid-crystal layer in the liquid-crystal display devices disclosed in Tokuhyo H4-502524 and the Japanese Unexamined Patent Application publication (JP-A) H6-130424 shift the phase of light by either .pi. (.lambda./2) or .pi./2 (.lambda./4) when no electric field is applied, and do not shift the phase of the light when an electric field is applied, the light emanating from the liquid-crystal layer striking an externally disposed circular polarizer, at which the light is either transmitted or reflected, in accordance with the degree of polarization of the incident light.
In the liquid-crystal display device disclosed in Tokuhyo H9-506985, although there is disclosure of one linearly polarized light component transmitted through an extended multilayer film striking a liquid-crystal cell, there is no disclosure with regard to the retardation at the liquid-crystal layer.
In the above-described combination reflective/transmissive type liquid-crystal display devices, which use a translucent diffusion sheet as a diffusion sheet for diffusing backlighting, the reflectivity of the diffusion sheet is low, resulting in poor display readability, in addition to the problem of low transmissivity.
In the above-described combination reflective/transmissive type liquid-crystal display devices, using frontlighting at the display, there is the problem of insufficient frontlighting intensity (efficiency), this causing poor readability.
Additionally, in the liquid-crystal display devices disclosed in Tokuhyo H4-512524 and in Japanese Unexamined Patent Application publication H6-130424, for the reason noted below, there is an extreme deterioration of readability of the liquid-crystal display and a great loss of contrast, thereby representing insufficient display quality.
More specifically, in the liquid-crystal display device disclosed in Tokuhyo H4-512524, because a circular polarizer directly visible from the outside is disposed outside the liquid-crystal layer, this polarizer being formed by a low-pitch cholesteric film with waveform-dependent reflectivity, approximately 50% of external light striking the circular polarizer is reflected, this reflected light directing striking the eyes of an observer, thereby greatly reducing the readability of the display.
In the case of the liquid-crystal display device disclosed in the Japanese Unexamined Patent Application publication H6-130424 as well, a color-selective layer that is directly visible from the outside is a circular polarizer made of a cholesteric liquid crystal, which in the same manner as in Tokuhyo H4-512524, reflects approximately 50% of the light that strikes it, thereby causing a great reduction in readability.
While there was in the past a dichroic circular polarizing layer (polarizer) of the light-absorbing type which absorbs light of one circular polarization direction and transmits light of another circular polarization direction, this was fabricated, for example, as a laminate of a .lambda./4 phase-shifting layer made of a dichroic polarizing material such as Polaroid (TM), which transmits light polarized in the transmission axis direction and almost completely absorbs light having a polarization direction perpendicular to the light transmission axis direction, the .lambda./4 phase-shifting layer causing either a lead or a lag of 45.degree. with respect to the light-absorption axis of the dichroic linear polarizing layer.
For this reason, a dichroic circular polarizer of the past functioned as a dichroic circuit polarizer only for light striking it from a dichroic linear polarizing layer, and did not function as such with respect to incident light from the opposite side.
Accordingly, it is an object of the present invention, in view of the above-described drawbacks in the related art, to provide a bidirectional dichroic circular polarizer that functions as a dichroic circular polarizer with respect to incident light from both directions.
Another object of the present invention is to provide a combination reflective/transmissive type liquid-crystal display device featuring high reflectivity and transmissivity, good readability, and which provides sufficient intensity, whether functioning as a transmissive-type display or a reflective-type display.
It is an additionally object of the present invention to provide a combination reflective/transmissive type liquid-crystal display device which has a simple configuration and which does not exhibit a loss of readability and a large decrease in contrast caused by external light, and which in the transmissive-type display mode particular, provides a great improvement in the utilization rate of light, and in the reflective display mode provides high contrast.